A simple typo fix: midi2_probe => midi2_ump_probe.

Fixes: febdfa0e ("ALSA: docs: Update MIDI 2.0 documentation for UMP 1.1 enhancement")
Link: https://lore.kernel.org/r/20230912075944.14032-1-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>